Due to the unavailability of an abundant supply of copper during periods of world conflict, electric generators which were built, for example, in the 1940's and early 1950's included aluminum or aluminum alloy field windings. The aging of such machines makes them prime candidates for extensive maintenance or complete rewinding as well as developing a need for effective methods for rewinding where necessary or desirable. That is to say, replacement of an original aluminum winding which is determined to be in good condition may not be necessary in that such windings may be reused with new field insulation installed. However, customer preference for copper may also require the replacement of the original aluminum windings. Moreover, where an original aluminum winding has been damaged, obtaining a suitable replacement alloy is not always possible or may involve substantial expense. Accordingly, replacement of original aluminum windings with appropriate copper substitutes is often necessary.
As will be appreciated by the artisan, such substitution to be effective and economically feasible is not as simple or as straightforward as it initially appears. For example, the use of aluminum field windings required the use of deep rotor slots in order to accommodate the approximately 80% increase in needed cross section for the windings. Additionally, since aluminum is approximately 35% as dense as copper, relatively thin or shallow rotor wedges were conventionally used. Furthermore, the use of a number of copper turns per coil, which is different than the number used in the original aluminum windings, would clearly have adverse effects on the field winding excitation requirements and thus require further modifications. Still further, the thermal performance of the rewound rotor should exhibit a performance at least equivalent to that of the original rotor with aluminum windings so as to eliminate extensive reevaluation and modifications pertaining to winding clearance requirements and cooling circuit capabilities.
Moreover, the mass of the substitute conductors should be approximately the same as the original windings in order to avoid over stressing of the wedges which serve to contain the windings within the rotor slots. In this regard, a substitute winding of copper, for example, may occupy only 50-60% of the original slot volume, while nevertheless weighing approximately the same as the original aluminum windings that have been replaced. However, prior attempts to simply replace the aluminum windings with copper located at the top of the existing rotor slots with the balance of the excess slot space occupied by fillers of insulating material, resulted in the rotor wedges having to restrain not only the copper winding, but also required restraint of the weight added by a stack of fillers. Thus, the addition of insulating fillers in the required quantities added significant weight and shear forces on the relatively thin rotor wedges. Such conditions resulted in over stressing both the rotor slot wedges, as well as the rotor teeth included in the rotor shaft drive mechanism. Still further, the fillers clearly present a significant barrier to the transfer of heat from the windings to the surrounding forged steel rotor, as well as serving as a barrier to cooling gases, such as air. Moreover, attempts to add conductor material for the purpose of reducing current density and hence a decrease in heating also serves to overload the rotor slot wedges and the rotor teeth.